1. Field of the Invention
The invention relates to a method for the production of defined plastic particles based on poly(meth)acrylate, particularly plastic particles which are stable in shape, in the particle size range 1-15 .mu.m.
2. Discussion of the Background
In practice, there is an increasing demand for plastics in the form of plastic particles which are stable in shape, with a defined, uniform particle size. The particle size range of 1-50 .mu.m and especially the narrower range of 1-15 .mu.m are of particular interest. Particles of this type find applications in many different ways, for example as spacers in displays, films, etc., as surface modification agents, as carrier materials in diagnostics, etc. The main interest, however, is in the sector of the optical industry, which can achieve different light effects, particularly with particles in this size range, with an exactly adjustable refraction difference to a given light matrix.
The state of the art with regard to the production of polymethacrylate (PMMA) particles is very comprehensive. A great number of works relates to the production of polymethacrylate dispersions in organic media, for example according to Jpn. Kokai Tokyo Koho 8179,111 (=Chemical Abstracts: 95: 188086w), cross-linked particles of polyalkyl (meth)acrylates with C.sub.6-20 alkyl groups or polyvinyl esters of C.sub.7-21 fatty acids with a diameter of 5-8 .mu.m are produced according to the pouring method, in Isopar G (aliph. petroleum oil). In addition, a large number of microfine, stable acrylate dispersions in organic media, for example for coating purposes, are described in, for example JP 62-220,564=Chemical Abstracts 108: 133526w. JP 61-181,856=Chemical Abstracts 106: 34795k describes an extremely microfine dispersion which is stable for &gt;6 months without sedimentation. A good overview of dispersion polymerization of methyl methacrylate in non-aqueous media is offered by the article by M. A. Winnik et al., in Makromol. Chem. Macromol. Symp. 10/11, 483-501 (1987).
Winnik et al. describe the influence of the solvent quality of the organosol medium on the polymerization speed of MMA or the influence of the solvent quality on the particle size, for example.
In general, hydrocarbons with a low boiling point, such as cyclohexane, heptane all the way to dodecane are described. As emulsifiers, many different types of block copolymer systems are used.
The requirements profile for polymer particles of the stated type, with a diameter of approximately 1-15 .mu.m, has been known for a long time; however, there was no advantageous, technically-practicable production method. In German patent application P 43 27 464.1, the production of polymethyl methacrylate particles which are stable in shape is described in cyclohexane as the solvent/precipitant. In this method, very uniform particles with a diameter of 2-10 .mu.m are obtained. According to knowledge available until now, a large number of boundary conditions had to be precisely adhered to.
The teaching of the stated patent application furthermore prescribes the use of percarbonates as initiators. There continues to be an interest in getting away from the various limitations of the stated methods. However, no reduction in product quality could be accepted.
The method of the present invention opens up this possibility in an excellent manner.